This invention relates to an all-season radial-ply pneumatic tire for passenger and light-truck vehicles. An all-season pneumatic tire is regarded as one which currently qualifies for a mud and snow (M&S) rating under the criteria for such rating established by the Rubber Manufacturers Association, Akron, Ohio. In general, such rating requires that the tire tread have full-depth grooves that are angled laterally toward the respective lateral edges of the tread surface for at least one-half inch and such grooves are required to have a width of at least 0.060 inch.
All-season tires are characterized by grooves that define blocks, and sometimes one or more ribs, in the tread surface. This results in a more aggressive appearance than that of a more conventional fully-ribbed tire and provides improved traction characteristics in snow and under wet conditions as compared to the conventional ribbed tire. All-season tires are distinguished from snow tires in that the former are required to satisfy all Federal Motor Vehicle Safety Standard 109 tests applicable to radial passenger tires in general, but the latter are not required to satisfy the provisions in such standards relating to high-speed test performance.
All-season tires customarily employ block elements in their tread designs. The block elements are surrounded by grooves typically having a width at the tread surface of at least 2 mm, although the tread design may include some narrower grooves or "sipes" to improve traction or appearance characteristics of the tire. The net-to-gross ratio of the tread of such tires typically is in the range from about 55% to 70%. The net-to-gross ratio, for purposes of this specification, is defined as the ratio of the net road-contact area of the tread surface divided by the total area of the tread surface (road-contact area plus void area) for a tire under normal inflation pressure. The net-to-gross ratio is measured when the tire is normally loaded at an inflation pressure normal for such load.
The use of blocks as elements in the tread of all-season tires tends to increase the noise level generated by such tires as compared to rib-type radial-ply pneumatic tires conventionally used in passenger and light-truck vehicle applications. Also, such blocks have a tendency toward irregular wear due primarily to their lack of stiffness in the circumferential direction of the tread. Such noise generation and irregular wear are accentuated at acute angles in the rubber blocks formed by intersections of groove walls.
As the rubber blocks enter the area of contact of the tread surface of the tire with a paved road surface, the entering portion of the block compresses and then, as the block passes through the mid-region of the contact area, becomes elongated causing the block to scrub the pavement. This can result in irregular wear. On the other hand, noise generation occurs when edges of the blocks impact the pavement. Noise levels produced by all-season tires, as compared to rib-type tires, are generally greater and are caused in large part by the impacts of block edges with the paved road surface.